1. Field of the Invention
The present invention relates to an apparatus for and a method of controlling a power train, and a storage medium in which a program for implementing the method is stored. More specifically, the present invention relates to a technology for controlling a power train that is connected to an automatic transmission, and that includes a driving power source whose output torque is boosted during a downshift.
2. Description of the Related Art
In a vehicle on which an automatic transmission is mounted in which one of a plurality of gears is established by selectively engaging a plurality of frictional engagement devices, a gear is established corresponding to a combination of the frictional engagement devices that are engaged. Accordingly, downshifts and upshifts are performed by bringing the frictional engagement device that is in an engaged state into a disengaged state, and bringing another frictional engagement device that is in a disengaged state into an engaged state.
In such an automatic transmission, the torque that the frictional engagement devices can transmit temporarily drops in the course of a shift. For this reason, there is a case where the output torque from the automatic transmission is increased stepwise as the shift operation progresses after the output torque from the automatic transmission temporarily drops. When the output torque is increased stepwise, a shock occurs. Technologies for increasing the output torque from a driving power source, that is, the input torque supplied to the automatic transmission during a shift, and thereby suppressing a drop in the output torque to reduce such a shock, are available.
Japanese Patent Application Publication No. 2004-316838 (JP-A-2004-316838) describes a shift controller for an automatic transmission that causes a motive power source, such as an engine, to perform a torque boost operation to make a shift operation progress as planned. The shift controller described in JP-A-2004-316838 includes: an upper limit value-calculating section for, when a shift is commanded, calculating an upper limit value of the torque from a motive power source that can be produced under a shift operation; a power source torque-calculating section for, when a shift is commanded, calculating the torque from the motive power source that the motive power source produces under the torque boost operation; an engagement pressure-setting section for setting, based on the upper limit value, a desired engagement pressure of gear-shifting friction devices whose state should be changed at the time of a shift when it is determined that the torque from the motive power source exceeds the upper limit value based on the signals from these sections when a shift is commanded; a torque-controlling section for, when the torque from the motive power source exceeds the upper limit value, controlling the motive power source so that the output torque from the motive power source is equalized to the upper limit value; and an engagement-controlling section for controlling the gear-shifting friction devices so that engagement pressure is equalized to the desired engagement pressure in the course of a shift when the motive power source torque exceeds the upper limit value. When it is determined that the motive power source torque exceeds the upper limit value in the course of a downshift, a speed of reduction of the engagement pressure of the friction devices (frictional engagement devices) that should be brought from an engaged state into a disengaged state is set based on the upper limit value of the motive power source torque. The friction devices are controlled so that the engagement pressure is reduced at a desired engagement pressure reduction speed.
The shift controller described in this official gazette compensates for the shortage in the torque boost amount due to the upper limit value by adjusting the engagement pressure of the gear-shifting friction devices. In this way, it is possible to certainly absorb and reduce the output torque shock immediately after a torque phase of a shift. Thus, it is possible to ensure a sufficient measure against the shift shock.
However, even when such a torque boost operation is performed during a shift as in the case of the shift controller described in JP-A-2004-316838, gradual reduction of torque capacity (engagement force) of a frictional engagement device in the course of a shift can cause a situation where the reduction in the output torque from the automatic transmission in an inertia phase of a downshift is not sufficiently prevented. As a result, the output torque can sharply vary in the course of a shift to cause a shock when, for example, the output torque from the automatic transmission varies stepwise in the course of transition from an inertia phase to a torque phase.